1. Field of the Invention
The invention relates to a current control circuit of a ring oscillator which is comprised of CMOS.
2. Description of the Prior Art
The ring oscillator circuit is comprised of a current control portion and a ring oscillator core portion. The ring oscillator core portion is comprised of odd number of inverters connected to each other in a ring shape and generates a clock signal. FIG. 4 shows a general block diagram of a conventional ring oscillator. FIG. 5 shows a detailed drawing of the conventional current control circuit. The current control circuit controls the clock oscillator frequency at the core portion 30 of the ring oscillator according to the input control current I. FIG. 6 is a detailed circuit diagram of the conventional ring oscillator core portions.
FIG. 7 explains an oscillation operation of the conventional ring oscillator circuit. FIG. 7 (a) shows a serial circuit of a constant current source and an inverter at a certain stage of the ring oscillator core portion in order to explain the oscillation principle of the ring oscillator. FIG. 7 (b) shows the delay time .tau. between an input signal and an output signal in each inverter. FIG. 7 (c) shows the relationship between the delay time .tau. and the current flowing in the inverter. FIG. 7 (d) shows the relationship between the oscillation frequency f and the current i flowing in the inverter.
An operation of the ring oscillator is explained below. In FIG. 7 (a), the inverter is constituted of P-type MOS transistor and N-type MOS transistor and driven by the current sources connected to upper and lower portions thereof. In FIG. 7 (b), the output signal OUT of the inverter is delayed from the input signal IN by the delay time .tau.. The delay time .tau. is a function of the control current i of the inverter. This delay time .tau. decreases exponentially according to the current increase without being proportional to the current i.
As shown in FIG. 4, the ring oscillator core portion 30 is comprised of a plurality of unit inverters arranged in parallel for n stages (odd numbered stages). The current sources 21-2n and 31-3n supply current i to the respective inverters (51-5n). These current sources are controlled by the input current I which is inputted from the input terminal 10 of the current control circuit portion 20. Assuming that the input terminal of the first stage inverter becomes H (logic level 1) during the current i flows in the inverter, L (logic level 0) is outputted to the output terminal 60 of the last stage inverter 5n after the delay time of n.tau. seconds. Since the output terminal is connected directly to the input terminal of the inverter 51, if the voltage of the input terminal becomes L, then voltage of both the output and input terminals becomes H after n.tau. seconds. Levels H and L are repeated in this way at the output terminal and then self-oscillation occurs. Since H and L levels are repeated for every n.tau. seconds, the oscillation frequency f is obtained such as f=1/2n.tau.. As shown in FIG. 7 (c), the delay time .tau. decreases when the current i flowing in the inverter increases, but its inclination becomes smaller when the current i increases. As shown in FIG. 7 (d), the frequency f therefore saturates when the current i increases.
FIG. 3 shows the relationship between the input control current I at the input terminal 10 of the current control circuit 20 and the output current I.sub.0 (or I.sub.10) from the current control circuit 20. FIG. 3 also shows the relationship between the input control current I and the output frequency f of the ring oscillator core portion. The dotted curve 101 in FIG. 3 shows characteristics of I.sub.0 of the conventional ring oscillator. Since the current control circuit 20 of the conventional ring oscillator carries out the linear current control as will be discussed later, the frequency characteristics of the whole ring oscillators show a non-linear curve as shown in the dotted frequency curve 104.
The non-linearity of this frequency characteristics becomes larger as the delay time of the inverter becomes shorter, that is, the current becomes larger. In other words, the shorter the delay time .tau. and the higher the using frequency becomes, the worse the linearity of the frequency becomes. On the other hand, the longer the delay time .tau. and lower the using frequency, the better the linearity of the frequency characteristics becomes.
In the conventional current control portion 20 in FIG. 5, the difference of the current (I.sub.1 -I), which is obtained by subtracting input current I from the current I.sub.1 supplied from the constant current source 21, flows through the drain of the transistor Q1. This current (I.sub.1 -I) is current mirrored to the transistor Q17. The current flowing in the transistor Q17 also flows in the drain of transistor Q16 simultaneously. The current flowing in this transistors Q16 and Q17 is current mirrored to the transistors Q21 and Q22 of the constant current source inside the ring oscillator core portion 30 via the output terminals 40 and 50, respectively.
In the conventional current control circuit, the input control current I has a linear relationship with the output current I.sub.0 (the current for controlling the constant current source in the ring oscillator core portion by the current mirror). Since the ring oscillator core portion 30 is controlled by the linear current, the frequency characteristics of the ring oscillator has non-linear characteristics as shown by the dotted line 104 of FIG. 3.
When said ring oscillator is used in the phase-locked loop (PLL), the non-linearity of frequency/current characteristics of the ring oscillator causes a variation of the cut-off frequency of the loop and a drift of the phase margin, which results in gain peaking or extreme gain decrease. According to the gain peaking or gain decrease, the follow-up to the loop becomes sensitive or impossible which makes the jitter drift. In PLL circuit, when the gain varies in the loops, the jitter drifts because of the phase characteristics of the loop (phase margin) and such.